This invention relates generally to dry paint transfer techniques, and more particularly to the use of these techniques to produce exterior plastic car body members or panels with a paint coat applied in a separate production operation which can eliminate or greatly reduce the painting steps carried out in the manufacturing operations of a conventional automobile production plant.
Automotive design and manufacture present a unique set of problems in the selection of materials and processes used in the manufacture of automobile bodies. As a form of transportation, automobiles are unique because most buyers want a vehicle to have a certain individual styling. A recent trend in the automobile industry is toward production of distinctive vehicles styled to attract specific groups of consumers. This change has required the car builder to shift production from a few models manufactured in large volumes to a larger number of more distinctive body styles. These developments have demanded from the manufacturer both styling flexibility and reasonable tooling costs for each body style.
For many years, car body members and panels have been made predominantly from sheet metal. However, car builders now generally recognize that future use of plastics for exterior car body members and panels, rather than sheet metal, may provide a solution to meeting the demand for more unique body styling and reduced tooling costs. With the increasing concern for weight-reduction, car builders have also turned their attention to the use of plastics as an alternative to heavier metal parts. For instance, certain exterior car body parts of many automobiles now on the road are made from lighter-weight plastic. These parts include bumpers, rocker panels, fender extensions, window and door moldings, and the like.
As reported in Plastics World, November, 1986, p. 30 et seq., a number of advanced corporate development programs now in process are also seeking solutions to the problem of commercially manufacturing from plastics the larger exterior car body panels such as hoods, roofs, deck lids, and in fact, the entire car body, if possible. The concept of making a production car body from a material other than sheet metal dates back to at least as early as the mid-1950""s, when the Chevrolet Corvette was first manufactured with a fiberglass body. Developments in plastic resin technology in recent years have resulted in more sophisticated plastic materials of higher impact strength than fiberglass. Polycarbonates are an example. These developments in plastics technology have caused many plastics manufacturers, for about the last decade or so, to seek solutions to the problem of commercially producing the entire car body at a reasonable cost from these later-developed high-strength plastic materials. Recent development efforts have also been directed toward producing plastic car bodies from various alternative plastics molding processes, including use of SMC (sheet molding compounds) and RIM (reaction injection molding) techniques.
Development of a production process for making exterior car body members and panels from plastics requires solutions to a number of technical problems. These parts must be manufactured at reasonable costs for tooling, assembly and materials. The end product also must meet certain quality requirements. For instance, the resulting car body panel must have structural capabilities such as impact strength and resistance to mechanical stress necessary to compete with sheet metal. It must also have a paint coat with a defect-free and durable exterior surface. An automotive quality paint coat must meet certain specifications for a large number of physical properties in order to be capable of use as an exterior automotive paint coat. These properties include gloss; distinctiveness-of-image; hardness; abrasion resistance; weatherability such as UV resistance; impact strength; thermal stability, namely, resistance to extreme high and low temperatures; gasoline and acid resistance; cleanability; adhesion to the underlying car body panel; resistance to water and humidity exposure; and hiding ability or opacity of the paint coat.
In the past, a conventional production process for applying an exterior paint coat to car bodies made of sheet metal has involved transporting the preformed auto bodies to application lines in the plant where the car bodies are dipped in paint, followed by transporting them to a separate location for baking the paint coat and waiting until the hardened paint coat dries thoroughly. Most paint systems today are acrylic enamels which are cross-linked into a hard, glossy, durable paint coat during baking. Following painting, the car bodies are transported back to the production plant for further assembly operations. The painting of plastic car body members has typically involved manually spray painting the plastic parts in a separate painting facility, followed by drying, and then transporting the finished parts back to the assembly operation. Conventional painting of sheet metal car bodies and plastic parts is expensive and is a significant factory problem with respect to environmental protection, workers"" safety, corrosion treatment, and waste disposal. It is estimated that about one third of the total capital investment in an automobile production facility today is involved in painting car body members and panels.
In recent years, metalized laminating techniques have been used for forming a reflective, weather-resistant metal surface on molded plastic automotive trim parts. These techniques have not yielded plastic trim parts with a paint-coated surface capable of exterior automotive use. Such plastic trim parts have experienced difficulties in maintaining reflectivity and avoiding surface defects under mechanical impact and environmental exposure.
New car body applications of molded plastic materials can develop if the manufacturer can find a way to commercially produce plastic car body panels with a paint coat having the durability and appearance properties necessary for exterior automotive use. Moreover, if in-mold painting or coating processes can be developed as an alternative to conventional auto painting, then auto assembly plants can be more compact, and capital costs and environmental and safety problems associated with conventional car painting at the factory site can be avoided.
Many corporate product development efforts have sought alternatives to conventional painting of molded plastic exterior car body panels and parts, but without any known success to date. A number of problems must be overcome to develop an economical production-type paint system and process for applying a paint coat capable of exterior automotive use for molded plastic exterior car body panels and parts so as to eliminate conventional spray painting operations. For instance, use of the cross-linked acrylic enamel paint systems which are commonly baked on the sheet metal car bodies to produce a tough, glossy finish cannot be used in painting plastic car bodies because of temperature limitations. One approach, which is the subject of this invention, involves developing a paint-coated laminate which can be used to replace the conventional painted finish on the exterior of an automobile body. The laminate is made from a paint coat applied to a casting sheet by paint coating techniques. The dried paint coat is then transferred from the casting sheet to a laminate panel by dry paint transfer techniques. The coating operation permits use of high temperature resistant paint systems capable of producing a tough, glossy finish. The laminate is later thermoformed into a complex three-dimensional shape and then bonded or integrally molded to an underlying plastic car body member or panel. Injection-cladding techniques can be used to manufacture a molded plastic part and simultaneously bond the laminate to the exterior of the molded plastic part.
A number of technical problems must be overcome in order to use such a laminate in a thermoforming and injection-cladding process, while maintaining a defect-free painted surface with high gloss and durability properties throughout the thermoforming and injection-cladding steps. For instance, the laminate must be heat and vacuum-formable into a complex three-dimensional shape without cracking, deglossing, stress whitening or creating other surface defects. A paint coat on such a laminate can require a substantial amount of pigment in order to provide the necessary color density or opacity and distinctiveness-of-image. However, it has been found that use of pigments in a paint coat can cause deglossing of the surface when a paint-coated laminate is thermoformed. Deglossing may even occur in a nonpigmented clear coat during thermoforming.
In addition to surface gloss requirements, the finished paint coat must be defect-free. Defects must not be produced by the thermoforming step, and the laminate also must be bonded or molded to the underlying plastic substrate in a manner that hides any defects that may be present in the substrate material.
Moreover, a finished paint coat may have a reasonably high surface gloss, but still not have the desired visual appearance known as distinctiveness-of-image. This property relates to the mirror-like character of the finish and is measured by the clarity of an image reflected by the finished surface. It is difficult, in a thermoforming operation, to produce an exterior automotive paint coat with a high gloss level and a high distinctiveness-of-image.
Durability properties are also critical in producing a paint coat capable of exterior automotive use. The paint coat must avoid exhibiting defects when exposed to mechanical impact and avoid deterioration of the surface from exposure to chemicals and to the weather.
A paint system that produces the toughness or hardness necessary for exterior automotive use also must have the elongation properties and heat resistance necessary to allow thermoforming of the paint coat into complex three-dimensional shapes without cracking, deglossing, producing stress lines or other surface non-uniformities, or otherwise degrading the finish. Large pigment levels also adversely affect the strength and alter elongation properties of the paint coat. In addition, reliable bonding of the paint coat to the laminate and bonding of the laminate to the underlying substrate material are essential.
Thus, the desired paint system should have a critical combination of many physical properties in order to produce a surface capable of exterior automotive use, while retaining desired surface characteristics after the laminating, thermoforming and injection-cladding or molding steps have been carried out. However, some physical properties tend to be mutually incompatible in such a process. For instance, a paint system may have good qualities of durability such as hardness, toughness, weatherability and the like; but the same paint system may not have sufficient elongation to be heat-formed into a complex shape without cracking or otherwise losing its durability properties. Other paint systems may degloss when heat-formed into a complex shape. Some paint systems have sufficient elongation to permit thermoforming into the desired complex shape, but they are too soft and therefore lacking in the necessary hardness or durability properties.
In summary, there is a need for an economical production process for manufacturing highly contoured molded plastic exterior car body members or panels with a laminated paint coat having both the durability and the gloss and other appearance properties sufficient for exterior automotive use. Laminating techniques for applying a paint coat to such a molded plastic part can provide a valuable alternative to conventional painting of exterior car body members. Certain properties, such as glass-smooth, defect-free surfaces and uniform paint coats, are better produced by laminating techniques than by conventional painting. Capital costs also can be reduced and environmental problems can be alleviated. Laminating techniques require, in addition, however, a paint system and processing techniques capable of producing and maintaining, throughout processing, the durability and elongation properties, opacity, gloss and distinctiveness-of-image levels, and defect-free surface necessary for exterior automotive applications. The present invention solves these problems.
Briefly, one embodiment of this invention provides a process for manufacturing a molded plastic article having a finished paint coat with surface properties meeting criteria for exterior automotive use. These include properties providing predetermined durability, gloss and other appearance characteristics in the finished paint coat. In this process, the paint coat is applied to a three-dimensionally shaped exterior surface of an intermediate laminate by a combination of paint coating, dry paint transfer-laminating, and thermoforming techniques. The thermoformed laminate then can be bonded to or molded to an underlying plastic substrate, for example, by injection-cladding techniques, with the paint coat maintaining properties sufficient for exterior automotive use throughout these processing steps. The invention is described herein in the context of a paint coat applied to the surface of a plastic car body member or panel, but the invention is also applicable to other articles of manufacture having a paint coat with properties similar to those required for exterior automotive use.
Considering the application of the invention to a process for making a plastic exterior car body panel, a paint coat comprising a synthetic resinous material is coated in thin-film form onto a flexible, heat-resistant temporary casting sheet. The paint coat is dried on the sheet sufficiently to harden it and transfer a predetermined exterior surface gloss level from the casting sheet to the paint coat. The paint coat may comprise a clear coat and a separate pigmented color coat. The clear coat and color coat may be formed as separate thin-film coatings which are dried and then bonded to one another. The color coat, in combination with the clear coat, provides a composite paint coat having the durability, gloss and other appearance properties necessary for exterior automotive use. The paint coat is transferred to a semirigid, thermoformable synthetic resinous backing sheet so that the paint coat provides the exterior surface of the paint-coated backing sheet. The laminate formed by the backing sheet and the bonded paint coat is thereafter thermoformed to produce a three-dimensionally shaped preformed laminate, while still retaining durability, gloss and other appearance properties necessary for exterior automotive applications. The thermoforming step can produce substantial elongation of the paint coat. The paint coat is capable of elongating from about 50% to about 150%, or more, during thermoforming, without significant loss of its exterior automotive durability, gloss and other appearance properties. In a subsequent injection-cladding step, for example, a synthetic resinous substrate material is injection molded behind the preformed paint-coated laminate to bond the laminate to the substrate. This forms a molded plastic article with a painted surface which retains the exterior automotive surface characteristics. The substrate can be an exterior body panel of a motor vehicle. The substrate material normally may have a substandard surface finish, but the backing sheet absorbs surface defects in the substrate material during the injection-cladding step so as to retain the minimum surface defect level and gloss required of an exterior automotive paint coat.
In one embodiment, the paint coat comprises a fluorinated polymer and acrylic resin-containing paint system with thermoplastic properties. The relative amounts of the fluorinated polymer and acrylic resin components in the paint coat formulation provide a sufficient level of resistance to deglossing and sufficient elongation for the laminate to be thermoformed into complex three-dimensional shapes while providing sufficient durability and appearance properties for the finished product to be useful as an exterior automotive body member or panel.
In one embodiment in which the paint coat comprises an exterior clear coat and an underlying color coat bonded to the clear coat, the clear coat can be formed from a fluorinated polymer and acrylic resin-containing coating. In this case, the clear coat can provide the majority of the durability, gloss and other appearance properties necessary for use as an exterior automotive paint coat. The color coat also can comprise a thermoplastic fluorinated polymer and acrylic resin-containing paint system. Other paint systems compatible with the clear coat and the backing sheet also can be used.
In one form of the invention, the fluorinated polymer component comprises polyvinylidene fluoride (PVDF), and the acrylic resin component can be a polymethyl methacrylate resin, a polyethyl methacrylate resin, or mixtures thereof, including copolymers thereof. One finished product having a surface capable of exterior automotive use is made from a paint system comprising from about 50% to about 70% PVDF and from about 30% to about 50% acrylic resin, by weight of the total PVDF and acrylic solids, exclusive of the pigment.
This invention also provides an exterior automotive quality paint coat having unusually high gloss and distinctiveness-of-image (DOI) along with other durability properties sufficient for exterior automotive use. This embodiment of the invention includes a thermoformable laminate having a paint coat bonded to it in which the paint coat, prior to casting on a casting sheet, was prepared as a solution of vinylidene fluoride in acrylic resin. The paint coat comprises at least a clear coat of vinylidene fluoride and acrylic resin coated from solution, which produces thermoformed laminates with high combined gloss and DOI levels. Good combined gloss and DOI levels have been obtained experimentally when the clear coat and pigmented base coat both are prepared as a solution of vinylidene fluoride in the acrylic resin. In one experimental test of a finished car body panel, a 60xc2x0 gloss level of greater than about 75 gloss units and a DOI approaching about 90 units were achieved.
Different forms of the invention are disclosed herein. One embodiment comprises a thermoformable laminate having a paint coat with properties capable of use in forming an exterior automotive finish on a plastic exterior car body panel. A further embodiment of the invention provides a process for applying a paint coat to an exterior body panel of a motor vehicle so that the paint coat in its finished form has the durability, gloss and other appearance properties necessary for exterior automotive use. Another embodiment of the invention comprises a pressure-sensitive adhesive-backed paint-coated laminate capable of use in automotive paint coat repair.
Thus, the present invention provides a process and articles of manufacture in which a laminated exterior paint coat ultimately forms an exterior finish on a highly contoured surface of a molded plastic article. The paint coat has a sufficient combination of elongation and resistance to deglossing to withstand thermoforming without disrupting the high gloss surface originally present, while retaining the durability and appearance properties sufficient to be useful as an exterior paint coat for a plastic car body panel.